This invention relates to opto-electronic scale-reading apparatus of the kind wherein a read head and a scale are supported for movement one relative to the other for the incremental reading of marks provided on the scale, the scale is illuminated by incident light, the read head is arranged to view the scale in a given direction, and the scale comprises an elongate member having said marks spaced in the direction of the length thereof, spaces being defined between the marks.
GB-932,481 discusses the difficulty of providing good contrast between the marks and spaces at a metallic surface and describes a scale wherein such contrast is sought to be achieved by marks having highly reflective surfaces and spaces having diffusively reflective surfaces. However, the marks are applied to a flat surface of the scale member by a photo-resist process so that both the marks and the spaces have parallel surfaces and act as if they are substantially co-planar. This can still lead to difficulties in providing good contrast especially if the scale surface is contaminated.
GB-1,516,536 discusses the difficulty of contamination of the scale and describes apparatus wherein both the marks and the spaces are illuminated by diffuse light. However, such diffuse illumination necessarily reduces contrast, and contamination can reduce the contrast between the marks and the spaces still further because the marks and spaces would be at parallel substantially co-planar surfaces.
EP-0160811 describes a diffraction grating having regions of different periodicities with a view to forming diffraction fringes in correspondingly different positions. The different fringes define coded reference marks in an incremental opto-electronic scale-reading apparatus. The grating is a phase grating formed by a square or by a sawtooth profile illuminated by coherent light but the grating lines extend in the direction of the length of the incremental scale and do not themselves form the basis for an incremental count of position.